Having a new UI gives the consumer that the new version they actually purchased is actually different. It could have all the benefits in the world under the hood, but if nothing the user sees changes, the user won't care and probably will not buy it. Hence, changing even just the colors gives the user the perception that this indeed is a new version.

I was a beta tester for Inrix Traffic's new Android version and the UI change from 3.x to 4.0 was immense, but added so much to it and they really listened to thier testers/customers. It's a very good app made a lot better by the UI change and feature set change.

and speaking for windows 8, the look and feel grew on me. I like how solid and professional the new explorer UI is.

I'd rather UI changes every version, than none at all. Do you know how horrifying it is to look at new apps that have decades old UIs? Many nerd computing tools suffer this fate (Wireshark, for example), which make them disgusting to run anymore.

my current media player UI and it works well with metro.... but this is not the default skin

I made the compilation and the artwork[snip]

What player is that?I'm still using Winamp as it plays FLAC and my whole collection is in FLAC. every other player I have used I don't like. I'm stuck with winamp but I hate the UI. I used to use Jaangle but its UI is even worse.